In the battle against cancer, one approach receiving significant interest is the utilization of the patient's own immune system. Because of this, a key goal is to induce robust anti-tumor immune responses capable of effective tumor killing. While recent immunotherapeutic approaches have shown great promise, a major barrier to their efficacy is the presence of a suppressive subset of T cells named Foxp3+ regulatory T cells (Tregs). Tregs are thought to inhibit antitumor immune responses, and are often found at high densities in solid human tumors where their presence has been correlated with poor patient survival. Thus, many therapies target Tregs for depletion in an attempt to induce robust anti-tumor immune responses. However, such approaches are often limited by unintended adverse immune responses, as Tregs are also critical for inhibiting unwanted autoimmunity. In order to develop more specific and effective ways of targeting Tregs in the context of cancer, it is the long-term goal to understand the basic rules by which these cells operate. Several critical gaps exist in our understanding of how Tregs inhibit autoimmune attacks against solid organs; knowledge which would be beneficial for understanding how immune responses against a tumor are regulated. The specific objective of this proposal is to elucidate the functional roles Tregs play in the prevention of autoimmune attack against solid organs and to understand how these mechanisms relate to the inhibition of anti-tumor immunity and promotion of disease progression in the context of prostate cancer. Recent findings have demonstrated the existence of a subset of Tregs, termed Aire-dependent Tregs, which may be inherently adept at regulating immunity against tissue-specific antigens found in various organs throughout the body. Since these recent studies where one of the first to demonstrate the existence of this population, experiments rigorously evaluating the function and prevalence of Aire-dependent Tregs have yet to be carried out. The central hypothesis of this proposal is that Aire-dependent Tregs serve critical roles in the prevention of autoimmunity to solid organs and contribute to the regulation o prostate tumor immunity and subsequently its progression. The rationale for the proposed research is based upon the reasoning that most solid tumors develop from within solid organs, evolving in the context of normal tissue. Thus, if Aire-dependent Tregs are critical for preventing autoimmunity against solid organs, then one would anticipate organ-specific autoimmunity and anti-tumor immunity to be regulated by similar mechanisms. This hypothesis will be tested by pursuing the following two specific aims: 1) to determine the functional role of all Tregs, including the proposed new subset, in the inhibition of multi-organ autoimmunity, regulation of prostate tumor immunity and modulation of prostate tumor progression; and 2) to determine the prevalence and distribution of Aire-dependent Tregs throughout the body. The impact of the proposed research for the tumor immunology field will be two-fold, as the characterization of a novel Treg subset could lead to a variety of new areas of investigation, while the rigorous evaluation of the functional impact of all Tregs on prostate tumor progression are expected to be widely translatable to other solid tumor cancers. Ultimately, these studies are anticipated to lead to a more thorough understanding of the biology of Tregs and the identification of novel targets for the generation of new immunotherapeutic approaches for the treatment of cancer.